The present disclosure relates to a radiographic image processing apparatus, method and program for performing noise removal processing on a radiographic image.
Conventionally, in diagnosis using radiographic images of a subject, the radiographic images are displayed on a display device, such as a liquid crystal monitor, or output on a film as a hard copy after various kinds of image processing are performed on radiographic images obtained by radiography to make the radiographic images suitable for diagnosis. Here, radiographic images have a problem that noise, such as quantum noise of radiation, is noticeable in a low density area, in which a radiation dose is low. Therefore, as image processing for radiographic images, various methods for performing noise removal processing for suppressing or removing noise included in the radiographic images have been proposed. Further, various methods for improving the image quality of radiographic images by changing the contrast of the radiographic images have also been proposed.
Incidentally, noise included in a radiographic image and the contrast of the radiographic image vary depending on a radiography condition, a subject condition and detector characteristics, which are the characteristics of a detector for detecting radiation that has passed through a subject. Therefore, image processing is performed on the radiographic image based on the radiography condition, the subject condition and the detector characteristics. Accordingly, a high-image-quality radiographic image, in which noise has been appropriately removed and the contrast has been adjusted based on the radiography condition, the subject condition and the detector characteristics, is obtainable.
Meanwhile, while a radiographic image of a subject is radiographed, especially if the thickness of the subject is large, the amount of primary radiation, which passes through the subject without scatter within the subject and is detected by a radiation detector, is small. Further, the energy of radiation that has passed through the subject becomes relatively high. Therefore, the ratio of scattered radiation becomes high, and noise is more noticeable than signal values. Further, if the thickness of the subject is large, the amount of scattered radiation (hereinafter, referred to as “scattered radiation”), which is radiation that has scattered within the subject, becomes large, and the contrast of an obtained radiographic image becomes lower. Therefore, various kinds of techniques for changing the degree of image processing based on the thickness of a subject have been proposed. For example, Japanese Unexamined Patent Publication No. 2003-284713 (Patent Document 1) proposes a technique for lowering parameters of a contrast and frequency emphasis processing if a body thickness, which is the thickness of a subject, is small, and increasing these parameters if the body thickness is large. Further, Japanese Unexamined Patent Publication No. 2011-239804 (Patent Document 2) proposes a technique for performing noise removal processing and contrast emphasis processing based on a body thickness. Japanese Unexamined Patent Publication No. 2010-005252 (Patent Document 3) proposes a technique for performing image processing so that a bone region included in a radiographic image is emphasized as a body thickness is larger.